Radioactive materials undergo radioactive decay, emitting nuclear radiation. Explain what is meant by the term 'radioactive decay' and describe the changes that occur in the nucleus of an atom when it emits an alpha particle and when it emits a beta particle.

Eduqas A-Level Physics — 6.1 Radioactivity · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (5 marks)

Radioactive decay is the spontaneous, random emission of nuclear radiation from an unstable nucleus.

When an alpha particle (²He) is emitted the nucleus loses 4 units of mass number and 2 protons, so A decreases by 4 and Z decreases by 2.

When a beta particle (an electron) is emitted a neutron in the nucleus changes to a proton, so the mass number remains unchanged but the atomic number increases by 1.

Examiner tips

  • Use the word ‘spontaneous’ and ‘random’ for decay; include the change in A and Z for alpha, and only Z for beta.
  • Show the particle symbols (α, β⁻) to demonstrate understanding of the emission.

Common mistakes

  • Confusing the change in mass number for beta decay; it actually stays the same.
  • Mixing up the direction of change for the atomic number in alpha decay (it decreases, not increases).

Mark scheme (5 marks)

  1. Radioactive decay is the spontaneous / random emission of nuclear radiation from an unstable nucleus
  2. When an alpha particle is emitted, the mass number of the nucleus decreases by 4
  3. When an alpha particle is emitted, the atomic / proton number decreases by 2
  4. When a beta particle is emitted, the atomic / proton number increases by 1
  5. When a beta particle is emitted, the mass number stays the same / does not change

Key terms in this question

radioactive decay · alpha particle · beta particle

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